1. Field of the Invention
The present invention relates to a mobile communications system, a communications control method, a mobile terminal device, a control method, and a recording medium. More particularly, it relates to a mobile communications system using W-CDMA (Wideband Code Division Multiple Access) mode.
2. Description of the Prior Art
Conventionally, mobile communications systems including portable telephones have been used effectively by users. Regarding third-generation mobile communications systems, IMT-2000 (International Mobile Telecommunication 2000) has been proposed as a global standard. In Japan, it has been stipulated as W-CDMA (Wideband Code Division Multiple Access) mode.
The W-CDMA mode covers a wider band than CDMA (Code Division Multiple Access) mode which is used in cellular systems wherein mobile stations conduct communications simultaneously in the same frequency band. The W-CDMA mode has the advantages of improving multipath resolution and reception characteristics and increasing the number of users who can communicate in the same band. Configurations of W-CDMA mobile communications systems are disclosed in “W-CDMA Mobile Communications modes” published by Maruzen Co., Ltd. (Jun. 25, 2001). FIG. 4 is a block diagram showing a configuration of a W-CDMA mobile communications system.
In FIG. 4, reference numeral 1 denotes a user equipment (UE) reference numeral 2 denotes a radio access network device which composes a radio access network (RAN), and reference numeral 3 denotes a core network (CN). The CN 3 is connected to a fixed network/mobile communications network 4 and the Internet 5. The radio access network device 2 is connected to the CN 3 and comprises a base transceiver station (BTS) 21 and radio network controller (RNC) 22. The UE 1 connects wirelessly to the BTS 21. The RNC 22 controls the BTS 21.
The CN 3 comprises a circuit switched (CS) domain 31 for switching control in relation to the fixed network/mobile communications network 4 and a packet switched (PS) domain 32 for switching control in relation to the Internet 5. The CS domain 31 comprises a mobile-services switching center (MSC) 33 and a gateway device 34. The PS domain 32 comprises a servicing general packet radio service support node (SGSN) 35 and a packet gateway device 36.
In this mobile communications system, location registration areas which correspond to BTSs are predefined. Mobility management for each communications network such as the fixed network/mobile communications network 4 or the Internet 5 is carried out by storing location information—which indicates in what location registration area the UE 1 exists—in a database which is called a home location register (HLR) (not shown) and is connected to the MSC 33 and the SGSN 35.
If the UE 1 is turned on in a location registration area or if the UE 1 is moved with its power off from a location registration area and is turned on in a new location registration area or if the UE 1 is taken from a location registration area to a different location registration area, location information must be registered or updated.
Operations for registering and updating location information have been defined in common specifications “3GPP TS 23 Series” standardized by 3GPP (3rd Generation Partnership Project). Registration and updates of location information according to the conventional mobile communications system will be described based on these specifications. FIG. 5 is a sequence diagram showing operations of an attach process and location registration process in the conventional mobile communications system. In FIG. 5, the UE 1, radio access network device 2, MSC 33, and SGSN 35 are identical to those in FIG. 4. An attach process is the process of notifying a network (communications network) that the UE 1 is turned on and is receive-ready.
If the UE 1 is turned on in a location registration area or it is moved with its power off from a location registration area and turned on in a new location registration area (hereinafter these operations will be referred to collectively as an “Attach Request”) or if the UE 1 is taken from a location registration area to a different location registration area (hereinafter this operation will be referred to as a “location registration request”), location information must be updated for mobility management in relation to each communications network—the fixed network/mobile communications network 4 and the Internet 5.
Referring to FIG. 5, first in a step S50, an RRC (Radio Resource Control) connection for radio communications is established between the UE 1 and radio access network device 2 (i.e., the RAN). This is a connection for radio communications between the UE 1 and radio access network device 2 and complies with a protocol which stipulates radio interfaces between UE 1 and radio access network devices 2 in Layer 3 (Network Layer) defined by the OSI (Open Systems Interconnection) model.
The process of establishing an RRC connection is started upon a request from an upper layer of the UE 1 or a call request from the communications network. In the case of a request from an upper layer of the UE 1, the process is started by an RRC Connection Request signal sent from the UE 1 to the radio access network device 2, RRC Connection Establishment signal sent from the radio access network device 2 to the UE 1, and RRC Connection Establishment Complete signal sent from the UE 1 to the radio access network device 2.
Next, in a step S51 of “Initial Direct Transfer [CS domain]”, the UE 1 transmits a Location Update Request signal to the MSC 33, i.e., the CS domain 31 with circuit switching capabilities for the fixed network/mobile communications network 4, requesting the MSC 33 to update the location information for mobility management in the fixed network/mobile communications network 4. Then, in a step S52 of “Downlink Direct Transfer”, the MSC 33 transmits a Location Update Accept signal to the UE 1, informing the UE 1 that the location information for mobility management in the fixed network/mobile communications network 4 has been updated.
Then, in a step S53 of “RRC Connection Release”, the radio access network device 2 transmits an RRC Connection Release signal to the UE 1. After receiving the signal, the UE 1 transmits an RRC Connection Release Complete signal to the radio access network device 2 in a step S54, and each of the UE 1 and the radio access network device 2 enters an idle state.
Next, in order to update the location information for mobility management in the Internet 5, an RRC connection for radio communications is established again between the UE 1 and radio access network device 2 (i.e., the RAN) in a step S55. Then, in a step S56 of “Initial Direct Transfer [PS domain]”, the UE 1 transmits an Attach Request signal to the SGSN 35, i.e., the PS domain 32 with packet switching capabilities for the Internet 5, requesting the SGSN 35 to update the location information for mobility management in the Internet 5. Then, in a step S57 of “Downlink Direct Transfer”, the SGSN 35 transmits an Attach Accept signal to the UE 1, informing the UE 1 that the location information for mobility management in the Internet 5 has been updated.
Thereafter, in a step S58 of “RRC Connection Release”, the radio access network device 2 transmits an RRC Connection Release signal to the UE 1. After receiving the signal, the UE 1 transmits an RRC Connection Release Complete signal to the radio access network device 2 in a step S59, and each of the UE 1 and the radio access network device 2 enters an idle state.
In this way, with the conventional mobile communications system described above, if an attach request or a location registration request is made for mobility management in the fixed network/mobile communications network 4 (CS domain 31) and mobility management in the Internet 5 (PS domain 32), mobility management information of only one domain is updated on an RRC connection between the UE 1 and radio access network (RAN) device 2 and the RRC connection is released once, upon completion of the update process. Then, an RRC connection is established again between the UE 1 and radio access network device 2, and mobility management information of further one domain is updated, and upon completion of the update process the RRC connection is released. That is, each RRC connection is used to update the location information for mobility management only in one domain.
However, in the conventional mobile communications system, when an RRC connection is established between the UE 1 and radio access network device 2, the location information for mobility management only in one domain is updated, specifically, a location registration process of IMSI (International Mobile Subscriber Identity) or GPRS (General Packet Radio Service) is performed. Consequently, location information is updated frequently. This presents a problem that while location information is being updated, the frequency of communication between the UE 1 and radio access network device 2 is increased, resulting in the inability to use radio waves (radio resources) efficiently.
particularly, the magnitude of the above problem depends on the size of the location registration area. That is, it is theoretically possible for the location registration area described above to cover the entire service area of the BTS 21 or to be divided into cells, the smallest unit. In such a case, too small location registration areas will cause the UE 1 to move among location registration areas frequently, increasing the number of attach requests or location registration requests made by the UE 1 and resulting in an increased number of updates to location information in the CN 3. Therefore, if mobility management information of only one domain is updated on an RRC connection, the frequency of communication between the UE 1 and radio access network device 2 will be increased, causing a problem of traffic congestion.
Furthermore, since the UE 1 judges whether it is located in the area within reach of radio waves from the BTS 21 by measuring the field strength of a control signal received from the BTS 21, if field strengths of received control signals around the boundary between areas formed by BTSs are intermingled, the UE 1 passing through the boundary will determine that it has moved into overlapping location registration areas of the BTSs and will make location registration requests one after another. In such a case, if mobility management information of only one domain is updated on an RRC connection, the frequency of communication between the UE 1 and radio access network device 2 will be increased, resulting in a waste of resources.